The invention relates to a display device having at least one electrically controllable display element. In particular, the invention relates to a display device which uses the effect of electrowetting.
It is known to realize displays with the aid of colored liquids which can be displaced between positions which are not visible and those which are visible for a viewer. An efficient means for displacing liquids is the so-called electrowetting wherein a voltage is supplied to an electrode array which is immediately adjacent to a liquid droplet, so that an electric field influences the liquid droplet causing that surface energy and therefore surface tension of the liquid increases. Depending on the configuration this can lead to spreading of the liquid on the electrode or covering the same completely, depending on which geometry and surface nature the electrode comprises and in which relative position liquid and electrode are located also with respect to the earth gravity field. It is known to provide the electrodes with a hydrophobic coating in order to achieve a maximum effect between the state of the liquid when the electrode is not supplied and when the electrode is supplied. With the aforementioned means it is already possible to realize a droplet movement without reverting to further functional elements.
Varying the surface energy incurs in case of a single droplet a variation of the contact angle at which the droplet wets the surface. The interrelation between fields strength and variation of contact angle is described by the Lippmann-Young equation and is known to the skilled person.
By a local variation of the electric field strength, therefore, the local surface energy of the liquid may be modified and thus, the geometry of a droplet can be changed locally. In particular, by generating a locally increased surface energy, a movement of the droplet may occur by extending preferably in the corresponding region which is characterized by presence of an electric field. Inversely, liquid is dragged from the influenced part of the droplet due to the surface tension out of the region where it is not influenced and therefore comprises a lower surface energy, since it tends in this region towards a geometry which is low-est in energy, namely that of a sphere. A net transport of liquid from the region of low field strength in the region of high field strength results thereby.
Based on these principles, a plurality of different display devices has already been realized. Commonly, it was always the objective to produce an electronically controllable bistable display element which selectively assumes one or the other state. Thus, it is for example known to reciprocate a liquid between a first and a second volume which are connected via a duct by means of the effect of electrowetting, wherein each volume comprises an associated electrode, so that the conveying direction respectively results from the ratio of field strength between both electrodes. Preferably, a potential is supplied respectively only to that electrode in which direction the liquid is to be conveyed.
U.S. Pat. No. 5,956,005 A discloses a display device having at least one electrically controllable display element each of them comprising a plurality fluidically closed volumes in which exactly one liquid having an electrically conductive or polar fraction and a non-polar fraction is present. Further, visible subvolumes are disclosed which are flat and stacked upon another.
An electrowetting based display device is known from WO 2009/036272 A1 wherein each display pixel is associated to a pair of electrodes forming a capacitor, by means of which a liquid which is located in the visible volume of the pixel, can be charged with an electric field. A backflow of the polar or electrically conducting liquid into a reservoir which is located in viewing direction to the display element below the visible display area is achieved by adjusting the geometries of the visible display volume and that of the reservoir such that the polar or electrically conducting liquid, if no electric field is supplied thereto, is forced into the reservoir due to the Young-Laplace pressure.
If a multi-colored display device were to be realized by means of the aforementioned technology, this would only be possible with the aid of a plurality of pixel comprising differently colored liquids which are arranged side by side on the display area. However, it is not possible to operate the device according to WO 2009/036272 A1 such that a single pixel may assume more than two color states. In particular, it is also not possible to represent mixed colors of at least two differently colored liquids and by means of a single pixel.